TY - JOUR
T1 - Octahedral microporous phases Na2Nb2-x TixO6-x (OH)x·H2O and their related perovskites
T2 - Crystal chemistry, energetics, and stability relations
AU - Xu, Hongwu
AU - Navrotsky, Alexandra
AU - Nyman, May D.
AU - Nenoff, Tina M.
N1 - Funding Information:
This work was supported by the United States Department of Energy (U.S. DOE) Environmental Management Science Program (EMSP) (Grant No. FG07-97ER45674). Some of the research described in this paper was conducted at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the DOE’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.
PY - 2005/3
Y1 - 2005/3
N2 - A family of microporous phases with compositions Na2Nb2-x TixO6-x (OH)x · H2O (0 ≤ × ≤ 0.4) transform to Na2Nb2-x TixO6-0.5x perovskites upon heating. In this study, we have measured the enthalpies of formation of the microporous phases and their corresponding perovskites from the constituent oxides and from the elements by drop solution calorimetry in 3Na2O·4MoO3 solvent at 974 K. As Ti/Nb increases, the enthalpies of formation for the microporous phases become less exothermic up to x = -0.2 but then more exothermic thereafter. In contrast, the formation enthalpies for the corresponding perovskites become less exothermic across the series. The energetic disparity between the two series can be attributed to their different mechanisms of ionic substitutions: Nb5+ + 02- → Ti4+ + OH- for the microporous phases and Nb5+ → Ti4+ + 0.5Vo.. for the perovskites. From the calorimetric data for the two series, the enthalpies of the dehydration reaction, Na2Nb2-x TixO6-x (OH)x·H2O → Na2Nb2-x TixO6-0.5x + H2O, have been derived, and their implications for phase stability at the synthesis conditions are discussed.
AB - A family of microporous phases with compositions Na2Nb2-x TixO6-x (OH)x · H2O (0 ≤ × ≤ 0.4) transform to Na2Nb2-x TixO6-0.5x perovskites upon heating. In this study, we have measured the enthalpies of formation of the microporous phases and their corresponding perovskites from the constituent oxides and from the elements by drop solution calorimetry in 3Na2O·4MoO3 solvent at 974 K. As Ti/Nb increases, the enthalpies of formation for the microporous phases become less exothermic up to x = -0.2 but then more exothermic thereafter. In contrast, the formation enthalpies for the corresponding perovskites become less exothermic across the series. The energetic disparity between the two series can be attributed to their different mechanisms of ionic substitutions: Nb5+ + 02- → Ti4+ + OH- for the microporous phases and Nb5+ → Ti4+ + 0.5Vo.. for the perovskites. From the calorimetric data for the two series, the enthalpies of the dehydration reaction, Na2Nb2-x TixO6-x (OH)x·H2O → Na2Nb2-x TixO6-0.5x + H2O, have been derived, and their implications for phase stability at the synthesis conditions are discussed.
UR - http://www.scopus.com/inward/record.url?scp=29044434141&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=29044434141&partnerID=8YFLogxK
U2 - 10.1557/JMR.2005.0100
DO - 10.1557/JMR.2005.0100
M3 - Article
AN - SCOPUS:29044434141
SN - 0884-2914
VL - 20
SP - 618
EP - 627
JO - Journal of Materials Research
JF - Journal of Materials Research
IS - 3
ER -